Comments - Trigger Overview

Okay, answering my own question: I should have known this, but the answer is to use a seperate "join table" that contains a pointer to both the originating record and the inserted record:

BEGIN
INSERT INTO general_journal SET
  -- other data as needed;
INSERT INTO `sales-general_journal` SET
  general_journal_ID = last_insert_id,
  sales_ID = NEW.ID;
END

Now, both records are available from either record by doing a LEFT JOIN on the `sales-general_journal` table.